Hiya Dulcineah/GrumpyStrumpet
sounds as though it's both of you in this property... apologies if I'm reading it incorrectly..
This situation sounds absolutley terrible, a neighbour in the same building should NOT have the keys to your place full stop. This needs to be fixed rightaway, I'd suggest changing the locks on the doors yourselves, getting a locksmith to do the change so the work is guaranteed and submitting the bill to the landlord.
All the time you are out of the property, who knows, this 'neighbour' might be snooping through all your belongings and that just doesn't bear thinking about!
As these people seem like the worst it's good you are keeping a diary of all events. Please be extra vigilant, take a good inventory of ALL your belongings and on a frequent basis check all is still there.
Have a check with the council, they may have some information, however, I suspect, that if the property is privatley owned, then the clauses and requirements can be over ruled (over the councils info) Usually (certainly for the apartment I own - and I'm also part freehold owner) external alterations would include a Satellite dish, Cable box etc (I was told I couldn't install a kitchen fan extractor for instance - so installed window mounted fans) so you would need to go along the routes of checking and re-checking the clauses in the contracts/lease/documentation.
Now for some practical advice in the meantime
1. With the amount of time and energy you'd need to expend just to see if you can get a hole drilled for Virgins broadband service - only you can decide if it's worth it compared to my 2nd point, which is -
2. Virgin are offering a 50 MB internet service to certain areas/customers at around £51/month. Is that the package you are requiring ? In your first post you mention as it's due to your flatmate's high volume usage. Would you mind breifly giving an overview on needing such a (for the UK!!) high speed service? this leads me to point 3...
3. I'm on a Home business (NOT home user) package from Be Broadband @ £21.50 per month and an 'upto' 24 MB service - real world average for me is between 11-15 MB/sec DL and 1MB UL. Static IP, and unlimited everything - absolutley fine for my requirements. You 'could' get an extra telephone line installed ( i think BT charge £125 for that) then ADSL enable that line from Be broadband and they will 'bond' them together giving you a significant bandwidth boost. You could opt to go with a different provider to provide a 'failover' feature, should one provider go 'offline' for some reason, you have the other one as backup. For that to work, you'd need a special box called a 'Balance Multi WAN Router' - basically like a normal router but with the capability to accept more than one input (WAN connection) have a look at this website
http://www.peplink.com/balance/tech-spec/- downsides to this whole arrangement is the extra costs of getting a line installed, and that currently is all up in arms anyway with the Virgin box scenario in the first place!
4. If the worst comes to the worst, have a look at
http://www.thinkbroadband.com/ and check their forums for a business level ISP (small business or professional package as they're known by) and see which ones take your fancy - granted they'll only be 'upto' 24 MB/sec packages, but for the amount of hassle you are currently experiencing, it might be the only way to go and save you dosh with solicitors fees, install fees etc etc.
Lastly, i know it's no real consolation, but Broadband provision in the UK is diabolical - as usual, the amount of red tape and politics puts the UK broadband way lower than many other countries - Japan and South Korea I think have been enjoying GIGABIT bandwidth for a number of years now (not everyone, but certainly available) and many other countries having broken the home 100MB bandwidth options at least 2-3 years ago - makes the UK's 8 MB service seem like a pittance!
Good luck in everything - get rid of the stupid neighbour holding the keys, if they cause a stink .. call in the cops that some of your stuff has gone missing.. and that *cough* the nasty neighbour has a key *cough*...

Cheers! DtM! West London & Slough UK!